(1) Field of the Invention
The present invention relates to a device for generating high frequency, high voltage RF signals, and more particularly, to an RF driver for providing high frequency, high voltage RF signals having well-defined envelopes and controlled amplitudes.
(2) Description of the Related Art
In systems for electron beam imaging and ion deposition printing, a print head having several closely spaced RF electrodes with a number of overlapping, transverse electrodes (called fingers) is commonly used to deposit charges on an imaging medium. Exciting the RF electrodes with high-voltage RF signals causes plasma to be formed at the intersection of the RF electrode and a finger. The potential of a finger determines whether charge is deposited on the imaging medium. The charge deposited on the imaging medium forms a latent image that can then be used to retain toner for transfer to a permanent recording medium such as paper. By controlling the application of the high voltage RF signals along with the potential of the fingers, a specific pattern of charges can be deposited on the imaging medium.
The accuracy and speed with which the pattern of charges is deposited upon the imaging medium depends, in part, upon the accuracy of the timing, duration and amplitude of the RF signals energizing the electrodes. To ensure proper deposition of charges, the RF driver must generate RF signals having an amplitude within a specified range for a specific ionization period. RF signals that are less than full amplitude or have a duration which is less than the ionization period are ambiguous as to the amount of ionization, if any, that they contribute and thus do not guarantee proper charge deposition. Therefore, the portions of RF signals that are less than full amplitude, such as the rise to, and the fall from the desired amplitude, should be eliminated or minimized. Furthermore, sharply defining the RF signal bursts in terms of both the rise to, and the fall from the desired voltage, enables the bursts to be applied in a more rapid succession.
In existing systems for electron beam imaging and ion deposition printing, the driver used to generate RF signal bursts is typically a resonant circuit formed by the secondary inductance of a step-up transformer and the load capacitance of the RF line in the print head. The primary of the transformer is initially driven with an arbitrarily wide current pulse in order to charge the output capacitance on the secondary side which, in turn, results in oscillatory ringing of the secondary resonant tank circuit. With this type of driver, the initial drive pulse must be made large in order to provide an output bounded by a square envelope at startup. Feedback of the secondary voltage (or the reflected primary voltage) is then used to provide subsequent drive pulses to the transformer primary. These drive pulses occur at the resonant frequency and have the correct phase relationship with the secondary voltage in order to sustain oscillation.
While existing drivers provide RF signals having well defined envelopes and controlled amplitudes, they generally suffer from one or more drawbacks that limit the applications with which they can be used. One such drawback is that the operating frequency of the driver depends not only upon the inductance of the step-up transformer secondary and the RF electrode capacitance, but also upon the signal delay through the feedback network and associated control logic. The accumulated delay through the feedback network and control logic, which often is relatively large, makes it difficult to ensure operation near resonance, particularly with designs operating at higher frequencies.
Another drawback associated with existing RF drivers is the design of the step-up transformer. The step-up transformer is designed to provide a high voltage output signal while maintaining a (typically small) secondary inductance selected to ensure resonance at the desired operating frequency. The large air gap required in the core of the transformer to set the secondary inductance may lead to a large leakage inductance. This leakage inductance increases parasitic ringing in the driver as the primary current is switched and leads to EMI problems and complicates design of the feedback circuit. This drawback is further magnified by the trend to higher operating frequencies with large capacitive loads, which makes the design of the traditional transformer impractical.
Some examples of existing systems can be found in U.S. Pat. No. 4,841,313 to Weiner and U.S. Pat. No. 5,142,248 to Theodoulou et al. Portions of these examples may be briefly summarized as follows:
U.S. Pat. No. 4,841,313 discloses an RF drive network for providing RF power to an external load. The RF drive network includes a voltage to current amplifier fed by a power one-shot which triggers on a zero crossing feedback signal from the drive line.
U.S. Pat. No. 5,142,248 discloses an oscillation circuit having multiple separately addressable oscillator stages for applying RF voltage across capacitive loads. Each stage includes a power driver, a transformer, and a quench circuit. A stage is activated by providing an initial pulse of AC current through the transformer primary. A common feedback circuit coupled to each of the transformers detects negative zero crossovers of the current through the transformer primary to maintain actuation of the driver.